Abstract: The rewarding properties of social interaction are critical for the development of adaptive social relationships. Because the social behavior of males and females evolved in response to very different selective pressures the neural mechanisms mediating social behavior are likely sexually differentiated. A critical gap in our understanding of the mechanisms regulating the expression of social behavior is an understanding of the basic neural mechanisms underlying social reward, particularly in females. Dysfunctions in the mechanisms mediating social reward play a major role in many psychiatric disorders [1-3]. Furthermore, sex differences in the mechanisms mediating social reward likely contribute to the well known, but little understood sex differences in the prevalence of psychiatric disorders [10,11]. Therefore, the overall goal of this proposal is to investigate sex differences in the neural mechanisms mediating social reward. Although the neural circuitry mediating social reward has not been fully elucidated, an essential component is the mesolimbic dopamine system (MDS). Our lab has very recently shown that oxytocin (OT) receptors in the ventral tegmental area are essential for social reward in male hamsters. Several lines of evidence suggest the MDS is sexually differentiated and I now have strong preliminary data that OT injected into the VTA increases the rewarding properties of social interaction in males, but decreases the rewarding properties of social interaction in females. This proposal will critically test the overarching hypothesis that there are major sex differences in the neural mechanisms mediating social reward. More specifically, I propose that these sex differences in social reward are the result of sex differences in the effects of OT in the ventral tegmental area. The proposed studies will improve our understanding of sex differences in the mechanisms underlying social reward and will provide an outstanding opportunity for research training.